In order to allow a Mac with T2 Chip to boot from PMM USBBoot you need to allow the Mac to boot from external media:
- Turn on your Mac, then press and hold Command (⌘)-R immediately after you see the Apple logo to start up from macOS Recovery.
- When you see the macOS Utilities window, choose Utilities > Startup Security Utility from the menu bar.
- When you're asked to authenticate, click 'Enter macOS password', then choose a local administrator account and enter its password.
In Startup Security Utility:
1. Set Secure Boot option to 'No Security'.
2. Set External Boot to 'Allow booting from external media'.